Hafnium scintillator array significantly improves high-resolution X-ray imaging

Researchers in China have developed a hafnium-based scintillator and silicon micropore array screen that could improve X-ray image resolution for medical diagnosis, industrial inspection, security screening and microelectronics

Researchers in China have developed a hafnium-based scintillator and silicon micropore array screen that could help to overcome a long-standing compromise in X-ray imaging between sensitivity and spatial resolution.

A research team led by Dr Jun’an Lai and Professor Dong Zhang from the Department of Radiology at Xinqiao Hospital, Army Medical University, worked with Associate Professor Peng He and Professor Xiaosheng Tang from the College of Optoelectronic Engineering at Chongqing University both in Chongqing, China, to investigate the high light yield of hafnium-based scintillators and to improve their imaging performance.

Since Wilhelm Röntgen discovered X-rays in 1895, X-ray imaging has become indispensable in medical diagnosis, industrial non-destructive testing, security screening and cultural heritage preservation. Modern X-ray imaging systems generally use either direct or indirect detection. Direct detection relies on semiconductor materials to convert X-ray photons directly into electrical signals. This approach can offer rapid response and high spatial resolution, but it is often costly and complex to manufacture.

Indirect detection, by contrast, uses a scintillator and photodetector architecture. In this configuration, X-rays are absorbed by a scintillator, which converts them into visible or ultraviolet light. A photodiode, charge-coupled device or complementary metal-oxide-semiconductor sensor then captures this light and converts it into an electrical signal. Because the technology is mature, compatible with established sensor platforms and comparatively low cost, indirect detection dominates the current X-ray imaging market.

The central difficulty for scintillator design has been the trade-off between thickness and resolution. A thicker scintillation screen absorbs more X-ray photons, thereby to improve signal strength and detection sensitivity. Yet a thicker screen also allows the emitted light to spread as it travels from the point of conversion towards the sensor. Multiple scattering and refraction events cause neighbouring image points to blur into one another, a process known as optical crosstalk. Conventional commercial scintillators have struggled to avoid this limitation.

The researchers designed and synthesised a series of hafnium-based organic–inorganic hybrid metal halides in which the organic cation chain length was varied systematically. The compounds were TTA₂HfCl₆, TEA₂HfCl₆, TMA₂HfCl₆, TPA₂HfCl₆ and TBA₂HfCl₆. Under 254 nanometre excitation, TMA₂HfCl₆ -- abbreviated to TMAHC – showed the strongest emission. Inductively coupled plasma mass spectrometry confirmed that the zirconium content in TMAHC was only 0.01 per cent which essentially ruled out a significant contribution from zirconium impurities.

Temperature-dependent photoluminescence spectra then revealed the mechanism behind the strong emission. Under 200 nanometre excitation, the 388 nanometre emission intensity increased as the temperature rose, a hallmark of thermally activated delayed fluorescence. The team further tested the mechanism through temperature-dependent photoluminescence decay lifetimes, excitation-power-dependent photoluminescence spectra, temperature-dependent Raman spectra and three-dimensional thermoluminescence measurements.

After it had established thermally activated delayed fluorescence as the key mechanism, the team evaluated the scintillation performance of TMAHC. The material achieved a light yield of 56,563.31 ± 1,250 photons per megaelectronvolt. It also reached a detection limit as low as 23.86 nanograys in air per second. An encapsulated scintillation screen based on the material showed minimal loss of light output and strong radiation stability, both of which are important for practical imaging systems.

However, high light yield alone could not solve the problem of optical crosstalk in a conventional planar scintillation screen. To address this, the researchers developed a silicon-based micropore array scintillation screen. In this structure, the scintillator was confined within microscopic pores, which helped to guide the emitted light and to limit lateral spread before it reached the sensor.

Zemax optical simulations confirmed the effectiveness of the crosstalk suppression strategy. In a screen with 25 micrometre pores and a filling factor of 95 per cent, the spatial resolution at a modulation transfer function of 0.2 reached 31.41 line pairs per millimetre. This figure was reported to exceed substantially the performance of gadolinium oxysulfide and thallium-doped caesium iodide screens, two established scintillator technologies used in X-ray detection.

“The ambiguity surrounding the luminescence mechanism severely hindered further performance enhancement and rational material design,” Dr Lai said.

The demonstration of thermally activated delayed fluorescence in this hafnium-based hybrid system clarified the physical process by which triplet excitons were converted to singlet excitons through reverse intersystem crossing and then contributed delayed fluorescence. This offered a clearer framework to interpret the luminescence behaviour of this class of materials.

“The extremely low defect density in TMAHC is a critical structural basis for its ultra-high light yield,” Professor Tang said. The study also indicated that the organic cation chain length influenced the singlet–triplet energy gap and charge-transfer efficiency by modulating distortion of the [HfCl₆]²⁻ octahedra. This finding points to rational design strategies to improve hafnium-based scintillators further.

“The development of silicon-based micropore array scintillation screens provides a low-cost, scalable solution for high-resolution X-ray imaging,” Professor Zhang said.

The team demonstrated imaging performance on an industrial chip and a biological model, which suggested potential use in high-precision non-destructive testing, microelectronics inspection, orthopaedics and dentistry.

The researchers noted that opportunities remain to improve the platform. The photoluminescence quantum yield of TMAHC could be raised further through more precise crystal growth and fewer surface defects. The filling factor of the micropore array, particularly for small pore sizes, also requires optimisation. In addition, the optical properties of the pore walls strongly affect crosstalk, which means that highly reflective metal or dielectric coatings could further improve resolution.

As understanding of the luminescence mechanism deepens and array structures improve, hafnium-based thermally activated delayed fluorescence scintillators could become an important platform for the next generation of high-resolution X-ray imaging systems.

For further reading please visit: 10.29026/oea.2026.250273